1. Field of the Invention
The invention relates to an apparatus and method for moistening a material web, and more particularly, to an apparatus for moistening a paper or cardboard web.
2. Discussion of Background Information
In paper making machines, problems frequently arise at the marginal regions (i.e., the edge regions) of a paper web. Such problems can arise in the headbox, where the fibers in these marginal regions are arranged in both the longitudinal direction (as in the center of the web), and in the width direction. The web tends to dry out more severely at the margins (i.e., the edges) of the web. At the surface of a drying cylinder, the region adjacent the web is hotter, because less heat dissipates from this region. Consequently, the edges of the web dry out more rapidly. This drying is also exacerbated by the fact that the marginal regions of the web are closest to the dry atmosphere of the paper-making facility and thus also give off moisture by convection.
Attempts to counteract this drying have been made by designing relatively complicated dryer cylinders, which apply heat to the edge regions of the web. Overheating at the edge regions of the web, however, leads to shrinkage and embrittlement in these marginal regions, which is disadvantageous for subsequent satinizing processes (i.e., the production of smoothness and/or gloss). Shrinkages lead to increases of density and thickness of the web edges.
Also, attempts to counteract these increases in density and thickness by applying less material at the headbox have led to undesirable results. Moreover, installation of a system of this kind is not possible for older facilities. The embrittlement no longer permits any reasonable smoothing in the calender, because the compressibility of the web is greatly restricted. The difference in alignment of the fibers at the web exacerbates this problem.
In German Patent Application No. DE 37 41 680, a related art moistening device is described, which extends over the full width of the web, with different transverse moisture profiles being selectable through a plurality of setting members.